Legendary duo

A Legendary duo is a pair of closely related Pokémon, either representing teamwork or opposition. Just as every main region has at least one Legendary trio, every main region, besides Unova, also has a Legendary duo. In the games, they are often found after the player gets past the Elite Four but often are not as important in the main storyline as the trios are. Duos or their component parts are featured in several of the movies as keepers of peace. Sometimes, pairs of Pokémon once thought to be Legendary duos are revealed to actually be Legendary trios upon the release of a third version; but, even within trios, the dynamic between two of the members is often very close to that of a duo. Kyogre and Groudon, Dialga and Palkia, Tornadus and Thundurus, and Reshiram and Zekrom are prime examples of this.

Latios and Latias take on counterpart availability in Pokémon Ruby, Sapphire, and Emerald as well as Pokémon HeartGold and SoulSilver and Black 2 and White 2, with Latios being available in Ruby, SoulSilver and Black 2, and Latias being available in Sapphire, HeartGold and White 2. In Emerald, after the player has defeated the Elite Four, their mother will ask if the Pokémon mentioned on TV was red or blue, and depending on that, one of the two will be roaming Hoenn. The other member of the duo will be found on Southern Island, accessible by Eon Ticket, in Ruby, Sapphire, and Emerald, and in Pewter City, through the Enigma Stone event, in HeartGold and SoulSilver.

Lunar duo

Cresselia and Darkrai

The lunar duo is a fan term for a Legendary duo of Sinnoh that consists of Cresselia and Darkrai. Cresselia and Darkrai are commonly seen as polar opposites, just like the aforementioned Johto bird duo of Lugia and Ho-Oh. Also, Cresselia and Darkrai reside on Fullmoon Island and Newmoon Island respectively, which are direct mirror images of each other. Cresselia and Darkrai are also known as the subconscious duo because of how Darkrai is said to give people nightmares, while Cresselia does the opposite (giving people pleasant dreams). Furthermore, Cresselia and Darkrai also appear in Pokémon Mystery Dungeon: Explorers of Time, Explorers of Darkness and Explorers of Sky as enemies.

Mortality duo

Xerneas and Yveltal on the cover of the X and Y official soundtrack

The mortality duo consists of Xerneas and Yveltal, which are found at the Team Flare Secret HQ in Pokémon X and Y, respectively. Xerneas is the Life Pokémon with the Ability Fairy Aura, while Yveltal is the Destruction Pokémon with the Ability Dark Aura. Xerneas spent a thousand years sleeping in the form of a tree before reviving and sharing eternal life. Yveltal absorbs the life force of all living things and turns into a cocoon when its life comes to an end.

Other duos

These duos are made up of Pokémon with a strong relation in the lore, but their stats and movesets tend to be fairly distinct and not equivalent; these duos can be said to be asymmetrical compared to the true duos.

Mew and Mewtwo

Mewtwo and Mew make up Kanto’s Legendary duo. They are both very elusive, if not unique, Psychic-type Pokémon. Mewtwo was cloned from Mew, meaning the two share DNA and many traits. In the games, however, they are usually treated as unrelated Pokémon, with Mewtwo's stats, level up moves and treatment in the Pokémon Stadium series being closer to that of the tower duo, while Mew's are closer to that of Celebi.

In Mewtwo Strikes Back, Mew embodied innocence and joy, but Mewtwo's experiences as it gained consciousness gave it a jaded and cynical view of humanity. The duality expressed between the two characters formed the basis of their conflict, but innocent Mew, with the help of Ash Ketchum, was able to sway Mewtwo's contempt.

Manaphy and Phione

Manaphy is the only Legendary Pokémon so far that is able to breed. The outcome however is a separate species, Phione. In a way the sea guardian duo are a perfect counterpart to the Mew duo; instead of an unnatural (through genetic engineering in a lab), stronger version, Phione is a natural (through breeding), weaker version of Manaphy; both also appear before the "original" in the Pokédex. Due to conflicting evidence from official sources, Phione's status as a Legendary Pokémon is uncertain; if it is not a Legendary Pokémon, the sea guardian duo is not a Legendary duo.